The direct measurement of the small proportion of testosterone that is not bound in the blood to carrier proteins such as sex hormone binding globulin, and so is available directly to the tissues. A more sensitive and specific test than serum testosterone.

Not often carried out at no cost, "free" here means the androgen (testosterone) in blood that is not bound to carrier proteins such as sex hormone binding globulin, so it's immediately available for action in the tissues. A more sensitive test than serum testosterone and likely to be increased in the polycystic ovary syndrome. An indirect measure of serum free testosterone, which can be performed instead.